1. Field of the Invention
The present invention concerns, a method and apparatus of precision electric-discharge machining.
One object of the invention is a thoroughgoing improvement in the known method whereby in spite of a wide range of such destructive actions as thermal distortion of the apparatus or vibrations and stress in the work, a high-quality product surface can be attained even with a relatively simple apparatus.
2. Description of the Prior Art
Work that can be electric-discharge machined to a surface roughness of 0.1 .mu.m with a very high-precision apparatus in a specially air-conditioned environment is known.
Polishing electric-discharge machined products electrochemically with the same wire cathode and with a powerful electrolyte is also known. When direct currents of approximately 3 A are employed, the higher surface elevations can be removed.
A very high-precision apparatus has drawbacks. It costs about three times as much as a moderately high-precision apparatus. Operating at temperature gradients of no more than 1 K over 24 hours, it cannot be accommodated in the average shop. Most applications on the other hand require only very smooth surfaces and not necessarily a very precise geometry. The electrochemical approach is extremely difficult to integrate into an electric-discharge machining system because the dielectric usually has a conductivity of 10 .mu.S/cm and it is almost impossible to separate the electrolyte from the dielectric. Furthermore, some materials that are markedly crystalline are almost impossible to polish because the agents attack different phases at different rates.
Other references will now be cited in supplement, although they do not necessarily address obtaining smooth surfaces with a moderately high-precision apparatus. German OS 3 929 111 A1 discloses a method of calculating displacement of an electric-discharge machining wire that does not require the entry of as much data to store offsets for roughing-out and finishing strokes. European Patent 0 312 056 A1 discloses a sensor that detects the position of an electric-discharge machining wire. K. Spanner and L. Dietrich, Feinste Positionierung mit Piezoantrieben, Feinwerktechnik & Messtechnik 87 (1979), 4, 181 describes using piezoelectric drive mechanisms for precision adjustment. The article describes laboratory instruments, however, and not machine tools. German OS 3 907 656 A1 discloses a precision tool with a system that senses the position of the work and with mechanisms that control it. German Patent 3 524 377 C2 discloses a system of guiding a wire through a C-shaped apparatus. German OS 3 813 306 discloses an electric-discharge machining apparatus with a device that detects surface-shaping errors by measuring the distance between a wire electrode moving along a prescribed reference path and the edge of the product. The frame of reference is either the apparatus itself or its coordinate system. Negative feedback is employed to compensate for the measured errors with high-precision electric discharge. The purpose of the sensor is to obtain highly precise contours and shapes and not to obtain smooth surfaces in moderately high-precision apparatus.